1. Field of the Invention
The present invention relates generally to the field of visual display systems for use in aircraft, flight simulators, etc., and more particularly to an apparatus for collimating a projected image at or near infinity, having high transmission of images incident thereupon.
2. Description of the Prior Art
Optical collimation apparatus capable of forming an image at or near infinity of an object or a plurality of optically superimposed objects have been known for some time. One such apparatus is described in U.S. Pat. No. Re 27,356, reissued May 9, 1972. As disclosed, an apparatus having a single spherically curved combining mirror is used as an image forming element. A primary image is directed at the convex side of the mirror, which transmits the image to a birefringent beam splitter array, positioned on the concave side of the mirror. The Array reflects the image back to the spherical mirror, collimating the image for viewing by an observer. Several polarizing filters in the light path selectively direct that part of the primary image which returns to the spherical mirror.
The apparatus taught in Re No. 27,356 may be assembled in compact size and light weight. However, due to the required multiple filtering and reflections of the primary image, transmissivity of the primary image is low. For example, each filtering and reflection of the primary image successively reduces the image's intensity by approximately one-half. The result is an ultimate transmission in the neighborhood of 0.5 to 10 percent of the original intensity of the primary image.
Another system, described in U.S. Pat. No. 3,940,203, issued Feb. 24, 1976, is a variation on the previously discussed Pat. No. Re 27, 356 which replaces the spherical mirror with a reflection-type holographic analog of a spherical mirror. Because the disclosed apparatus must employ a relatively large number of reflections and transmissions to properly control light which reaches the observer, this technique also suffer from low transmission of the primary image. By utilizing the improved reflection and transmission characteristics of the holographic element, efficiency on the order of 6 to 10 percent of an image's original intensity is achieved at best.
Thus, there is a present need in the art for an optical collimation apparatus with improved transmisivity of images. Further, since application of such optical collimation apparatus may include helmet-mounted display systems or other applications where size and weight are critical, there is a present need for such an optical collimation apparatus which is both compact and light-weight.